Post processing for metal additive manufacturing –
ensuring quality in every component
One of the icons of the American West, Wyatt Earp, said concerning his ability to win gunfights: “Fast is fine, but accuracy is everything.” What does metal additive manufacturing have in common with this legend of the O.K. Corral gunfight? In component production ‘getting it almost right’ isn’t good enough. Precision is as vital in component manufacturing as the ability to straight-shoot is in a gunfight.
To ensure the precision of every component, post processing is an essential part of additive manufacturing (AM). This is because the challenges that arise when melting and re-solidifying metal powder during the build process need to be mitigated during post production. The majority of additive parts will require one or more of the following post production processes:
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The metal additive manufacturing process creates a lot of heat in localised areas, due to the need to melt the powder using a laser. This can lead to stress points in the component when it cools. To reverse this, heat treatment is used to ‘normalise’ the material and put the grain structure into a uniform, relaxed state. Heat treatment can also optimise the properties of the component, adding features such as tensile strength or heat resistance.
Hot isostatic pressing (HIP) can also be used to reduce the porosity of metal parts. This process was originally developed for castings due to their higher porosity levels (up to 5% in some cases). Although laser powder bed fusion (L-PBF) produces much higher density parts (around 99.5% dense), the HIP process can also be used here to increase tensile and fatigue levels.
Electro discharge machining
Electro discharge machining (EDM) or wire erosion is a key part of the additive manufacturing process. During the build process, components are essentially welded to the build platform to stabilise them and EDM is used to remove them once printing is complete. An electrical current is passed through an electrode – usually a thin brass wire – and this is moved along the part, eroding the metal it touches and freeing the component from the platform.
As well as releasing parts from the platform using a straight cut, the EDM can also be used to produce high precision profiles within the components. As it is a no contact, no force process, EDM is ideal for use with delicate parts.
Support structures are used during the AM process to ensure a component’s integrity during printing. Automated or manual special finishing is then undertaken to remove the supports and ensure a consistent surface finish of around 15μm Ra remains across the part – the equivalent to a heavy shot blasted finish. Mating surfaces can subsequently be machined for ultimate precision. For those parts that require a finer surface finish for aesthetic or mechanical reasons, other finishing options are also available.
Some components are better suited to straight machining, others are ideal for AM but require some post process machining. This is because the mating surfaces of AM components have a general tolerance that is larger than that of normal machined components. This means that there is usually an element of machining required if the component is part of an assembly.
Some surfaces need to be finished to extremely high tolerances. In addition to EDM, precision surface grinding can be used to produce these very tight tolerance surface finishes.
As mentioned earlier, precision is key in the AM process and inspection plays an important role in maintaining the highest standards. There are challenges however in inspecting additive parts, due to their freeform geometries. This can be overcome using high-accuracy manual and automated co-ordinate measuring machines (CMM) and the portable Faro Arm CMM. There are occasions when the internal channels and features of a component cannot be inspected using traditional equipment and at these times, Frazer Nash works with several subcontractors to overcome these difficulties.
Frazer-Nash Manufacturing provides an expert end-to-end additive manufacture service for 3D printing custom metal parts. Find out more on our Additive Manufacturing process below.
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